1. Field of the Invention
The present invention relates to the field of increasing throughput in wireless local area network communications.
2. Related Art
A wireless local area network (WLAN) is a transmission system that provides for network access between electronic devices that are wireless stations using radio waves instead of direct cable connections. An AP is a wireless station with additional functionality. In the IEEE 802.11 standard, the WLAN consists of a number of basic service sets (BSS) that are joined by a distribution system into an extended service set (ESS). Within the ESS, a mobile unit or end station may roam at will while continuously maintaining a connection to the network.
Each BSS is controlled by an access point (AP). Each AP communicates with the end stations over the wireless medium in its BSS. The AP communicates with other APs and other nodes on the network via the distribution system. A function of the AP, among many others, is to relay network traffic from the end stations in its BSS through the distribution system to the destination. The destination of this traffic may be another end station in the same, or different, BSS, or the destination may be a node on a wired LAN (such as ethernet) connected to the distribution system. The AP provides this relaying function for multiple wireless end stations simultaneously. The relaying of traffic for multiple end stations results by the AP in an asymmetric distribution of the load entering a BSS.
The IEEE 802.11 standard uses a default or basic access mechanism implemented in the 802.11 Medium Access Control (MAC) layer. The 802.11 MAC layer protocol is called the Distributed Coordination Function (hereinafter referred to as “DCF”), that provides fair access to all users of the WLAN.
For example, in a BSS where the AP is relaying traffic for nine end stations, and where the traffic from each end station generates an equal amount of returned traffic to that end station, the IEEE 802.11 standard provides for the default DCF access mechanism to provide fair access to all users, including the AP, of the WLAN.
In the foregoing example, the available bandwidth of the BSS would be shared equally among the nine end stations and the AP, with each approximately receiving ten percent of the available bandwidth. In a sense, there is symmetric access to the network, where none of the end stations nor the AP have network access priority over the other users.
When using DCF access mechanism, a station that senses that the transmission medium is available is allowed to transmit over the WLAN. If the medium is not available, then the station waits for a certain time before trying to transmit again. This waiting period is a called a backoff period. The IEEE 802.11 standard and its variations uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism with a random backoff period for wireless connectivity between fixed, portable, and moving stations within a local area.
In another example, a scenario may exist where the end stations offer only half of the load to the BSS and the other half of the load is offered by the AP. If the total offered load of the AP and the wireless end stations is less that the available bandwidth of the BSS, the DCF access mechanism will be able to service the entire offered load.
However, as the offered load approaches the available bandwidth, the number of collisions caused by the use of the DCF access mechanism increases dramatically, adding significant delay to the delivery of the traffic. Eventually, the bandwidth lost to collisions plus the offered load from the end stations and the AP exceed the available bandwidth of the BSS and the delay suffered by the traffic approaches infinity. Thus, a need exists to provide a more efficient use of bandwidth through a wireless local area network, especially when the load conditions are heavy.